Organic synthesis has proven to be a highly effective means for preparation of molecules with useful biological activities which may be employed in the treatment of human, animal, and plant diseases. Combinatorial chemistry is a means of performing many organic syntheses concurrently or in parallel arrays, thereby increasing the rate at which compounds may be synthesized. In the preparation of biologically active molecules via organic synthesis or combinatorial chemistry, a multi-step organic synthesis is usually required. Each step consists of reacting various chemicals to produce a product which is normally purified before continuing with the next step. Purification is typically the most time consuming part of organic synthesis. The time spent on purification is especially critical in combinatorial chemistry since hundreds or even thousands of reactions are often carried out in parallel. Thus, methods which enable simple, rapid and readily automated purification are of value to the practice of both organic synthesis and combinatorial chemistry.
Solid-supported reagents which cause a chemical transformation of a compound in solution provide a convenient and rapid means of purification since they can be removed from the desired product by filtration. Solid-supported reagents are typically prepared by chemical reactions that attach individual molecules of the desired reagent to a preformed solid support either by covalent bonding or ionic interaction.
Solid-supported scavenging reagents also provide a convenient and rapid means of purification since they selectively react with certain components of a mixture in solution, thereby removing them from solution to the solid phase where they can be easily separated from the unbound components by filtration. A solid-supported scavenger may be used in one of two ways. First, it can be designed to selectively react with excess starting materials or other reactive impurities which contaminate the solution of a desired product. The resin and the sequestered contaminants are subsequently removed by filtration. Second, the scavenger resin can also be designed to selectively react with the desired product. With the product sequestered on the resin, any contaminants may be rinsed away. The product is then chemically cleaved from the resin in a purified form. This latter use of a scavenging resin is often referred to as “catch and release.” Solid-supported scavenging reagents are typically prepared by chemical reactions, which result in the covalent attachment of individual molecules of the scavenger reagent to a pre-formed solid support.
Aqueous suspension polymerization requires that any functionality on the monomers not be reactive with water nor promote dissolution of the monomer in water. This limits the range of monomers that may be utilized.
For a comprehensive review of the existing state of the art with respect to solid-supported reagents, solid-supported scavengers and solid phase organic synthesis, see Obrecht D. and Villalgordo J. M., Tetrahedron Organic Chemistry Series, Volume 17, Solid-Supported Combinatorial and Parallel Synthesis of Small-Molecular-Weight Compound Libraries.
A disadvantage with some solid-supported reagents and solid-supported scavengers is their low loading of reactive groups per gram of solid support. For instance, an isocyanate scavenger resin with 1.1 mMol of isocyanate groups per gram of resin has recently been described (Booth R. J. and Hodges J. C., Polymer-Supported Quenching Reagents for Parallel Purification, J. Am. Chem. Soc., 1997; 119:4882-4886).
The synthesis of block co-polymers by living free-radical polymerization has been described in the literature. For a recent and comprehensive review on living free-radical polymerization see Malmstroem Eva E.; Hawker Craig J., Macromolecular engineering via “living” free-radical polymerizations, Macromol. Chem. Phys., 1998; 199:923-935. One class of living free-radical polymerization reactions makes use of nitroxide reagents to initiate the reaction and cap the growing polymer chain. For example, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (OH-TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (oxo-TEMPO) and related 0-alkyl derivatives such as 1-Phenyl-1-(2′,2′,6′,6′-tetramethyl-1′-piperidinyloxy)-ethane (1-Phenylethyl-TEMPO), are useful nitroxide reagents in living free-radical polymerization. A variety of useful nitroxide reagents are described by Chang Hun Han. Marco Drache, and Gudrun Schmidt-Naake in Die Angewandte Makrornolekulare Chemie, 1999; 264:73-81. A variety of useful alkoxyamines and their performance in living free radical polymerization reactions are described by Didier Benoit, Vladimir Chaplinski, Rebecca Brnslau and Craig J. Hawker in The Journal of the American Chemical Society, 1999; 121:3904-20.
The use of 1-Phenylethyl-TEMPO in a living free-radical polymerization as previously described usually involves heating the 1-Phenylethyl-TEMPO above 123° C. to cause it to reversibly fragment into a TEMPO radical and a phenylethyl radical. When this is done in the presence of an excess of styrene, the phenylethyl radical initiates polymerization of the styrene. Each molecule of 1-Phenylethyl-TEMPO grows one polymer chain of approximately 10 monomer units, the terminus of which is capped with a TEMPO residue. Subsequently, it is possible to use the first polymer as an initiator for an even larger polymer. Heating of the first polymer with excess 4-bromostyrene causes continued polymerization to give a polymer of approximately twice the number of monomer units in two blocks, one of which is polystyrene and the other of which is poly(4-bromostyrene). The term “living free-radical polymerization” arises from the potential ability to start, stop, and continue polymerization reactions in repeated cycles. The polymerization reaction “lives” a long time since chain termination reactions that would “kill” the polymerization reaction are inhibited by the presence of the TEMPO radical.
Preparation of other functional polymers have been described in the literature. An extensive review of functional polymer preparations has recently been edited by A. O. Patil, D. N. Schulz, and B. M. Novac (Functional Monomers, Modern Synthetic Methods and Novel Structures, ACS Symposium Series 704, The American Chemical Society, 1997, 347 pages).
Insoluble solid supports may be chemically modified to contain multiple cyclic nitroxide sites which can act as initiators to radical polymerization has recently been described (PCT Publ. WO 00/78740). Solid-supported functional polymers prepared from these solid-supported initiators are also described. The macromolecular structure of these solid-supported functional polymers (termed “Rasta resins” because of the schematic appearance of the hair-like appendages that represent the new block polymer growth) allows properties such as greater solvent accessibility to reaction sites and higher loading levels of reagent functionality compared to known solid-supported reagents, solid-supported scavengers, and supports for solid phase synthesis. However, lengthy reaction times are required to generate those solid-supported functional polymers. Furthermore, only modest loading levels were previously achieved for such polymers.
Recently, numerous examples of microwave-assisted protocols for organic synthesis have been described in the literature. However, microwave technology has not been applied to living free radical polymerization strategies in solution or on solid support.